A secondary loop refrigeration system typically incorporates two different refrigerants to provide cooling. In FIG. 1 an exemplary secondary loop refrigeration system 10 is shown. The system includes a primary loop 12 that is a traditional direct expansion design using a phase change refrigerant (e.g., R-404A). A compressor 14 raises the pressure of the refrigerant in the primary loop 12, thereby causing the refrigerant to circulate within the loop. The high-pressure refrigerant passes through an expansion valve 16, which reduces the pressure of the refrigerant and thus causes it to absorb heat from higher temperature surroundings.
The primary loop 12 is typically restricted to a machine room (not shown), and a primary heat exchanger 18 (e.g., a chiller) is used to transfer energy from the primary loop 12 to a secondary loop 20. In most applications, the refrigerant in the secondary loop 20 is a benign single-phase fluid that is circulated by a pump 22 to individual heat exchangers (not shown). Secondary refrigerants are usually liquids, such as water, brine, propylene glycol, etc. The secondary refrigerant absorbs energy by increasing temperature as it passes through the individual heat exchangers in the refrigerated spaces 24. The warmer refrigerant is then returned to the chiller 18, rejecting the heat to the primary refrigerant.
Flow of a refrigerant in a secondary loop 20 is typically regulated by a fixed flow rate valve 26, which maintains a constant (fixed) flow of the secondary refrigerant in the secondary loop 20. In this manner, a constant amount of refrigerant can be circulated through the secondary loop 20 per unit of time. Further, the temperature of the secondary refrigerant is typically regulated by interrupting the flow of the refrigerant to the chiller 18, e.g., by turning the flow valve off when the temperature of the secondary refrigerant drops below a predetermined threshold, and turning the flow valve on when the temperature of the secondary refrigerant rises above a predetermined threshold. To turn the flow valves on and off, a solenoid 28 can be coupled to the flow valve, thereby enabling selective control of the flow valve state.
Secondary loop refrigeration systems can be advantageous as they allow the amount of primary refrigerant to be minimized. In addition, secondary loop refrigeration systems maintain the primary refrigerant in a plant room where it can be closely supervised (thereby minimizing the risk of inadvertent discharge of the primary refrigerant and reducing the amount of refrigerant that is required in the system). Further, due to simplified piping, advances in circulating pumps, and reduction in system components, secondary loop systems tend to have lower initial costs when compared to costs of equivalent standard direct expansion systems.